AN4001 Datasheet, PDF (1/7 Pages) M/A-COM Technology Solutions, Inc. – Application Note 300 Watt Class E Amplifier Using MRF151A

Application Note
AN4001
Application Note
300 Watt Class E Amplifier Using MRF151A
Rev. 01262010
BACKGROUND
Modern industrial applications for high-efficiency, switch-mode RF amplifiers include laser, plasma,
magnetic resonance imaging (MRI), and communications. The power levels and frequency of operation of in-
dustrial equipment used in these areas vary greatly. While plasma and heating applications tend to cluster at
13.56 MHz and 27.12MHz, laser and MRI applications tend to migrate towards 40 MHz, 80 MHz, and 128 MHz.
Power levels span the gamut from a few watts to hundreds of kilowatts.
The stability, reliability, and low R_DS-ON resistance of MACOM high frequency, RF, power MOSFETs
make them suitable for switch-mode amplifier applications. The MRF product line, which includes RF power
MOSFETS in the 1MHz-1GHz frequency range, has been a communication industry standard for more than 30
years. These devices are also used in many switch-mode amplifier applications and can yield much higher
power and efficiency levels than specified in the traditional class AB designs. This application note presents a
class E amplifier design based on MRF151A, a single ended power MOSFET, where it yields up to 300 watts at
81.36 MHz with better than 82% efficiency.
Class E amplifiers are well suited to industrial applications due to their simplicity and the high efficiency
which can be obtained at a single frequency or over a narrow bandwidth. In this type of amplifier the power
transistor operates as an on-off switch and, and in conjunction with the load network, it offsets the current and
voltage waveforms in order to minimize power dissipation and maximize efficiency [1].
THEORY
A simplified schematic of a class E amplifier is shown in Figure 1. It consists of a transistor, a shunt
capacitance C, a series LC circuit, a load R, and additional bias and input matching circuitry. The shunt capaci-
tor C can be made up by the internal output capacitance of the transistor or by a combination of internal and
external capacitances. The transistor in this case operates as a switch and drives the load network C, C_o, L_o,
R. The design of this load network is done such that the voltage and current through the drain of the transistor
are out of phase while power is delivered to the load resistor R. This offset implies that, ideally, no power is
dissipated in the transistor thus the efficiency is ideally
100%.
Vdd
According to [1] the design equations of the load
network are given by (1)-(5). The design equations are
derived by starting out with the drain voltage waveform
equation and imposing a set of constraints peculiar to
the ideal class E amplifier circuit. P_o is the output power
delivered to the load R given a supply voltage V_dd. V_Dpeak
is the peak drain voltage. Reactances X and B take into
Co
Lo
account the limited Q value of the inductor (Q = ωL_o/R).
This Q value is assumed to be in the 3 to 10 range. B is
the susceptance of the shunt capacitance C. X is a re-
actance added to the resonance tuned L_oC_o in order to
C
shape the voltage and current waveforms for optimum
R
class E operation.
Figure 1. Class E Amplifier Block Diagram
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